Vacuum pumps typically comprise an impeller in the form of a rotor mounted on a shaft for rotation relative to a surrounding stator. The shaft is supported by a bearing arrangement comprising two bearings located at or intermediate respective ends of the shaft. One or both of these bearings may be in the form of rolling bearings. Usually, the upper bearing is in the form of a magnetic bearing, and the lower bearing is in the form of a rolling bearing.
As illustrated in FIG. 1, a typical rolling bearing 10 comprises an inner race 12 fixed relative to shaft 14 of the pump, an outer race 16, and a plurality of rolling elements 18, supported by a cage 20, for allowing relative rotation of the inner race 12 and the outer race 16. The rolling bearing 10 is lubricated to establish a load-carrying film separating the bearing components in rolling and sliding contact in order to minimize friction and wear, and shield elements 22 are provided to resist seepage of lubricant out of the rolling bearing 10. A mounting arrangement for supporting the rolling bearing 10 within the pump comprises a radial elastomeric damping ring 24 positioned radially between the outer race 16 and a housing portion 26 for damping radial movement of the outer race 16, and an axial elastomeric damping ring 28 positioned between an end face of the outer race 16 and the housing portion 26 for damping axial movement of the outer race 16.
There are a number of problems associated with use of radial and axial elastomeric damping rings 24, 28 for attenuating vibrations within a vacuum pump.
Firstly, complex damping ring locations are also required for dynamic stability. A thin axial damping ring is required for beneficial axial to radial stiffness contribution, and precise location of a thin axial ring requires complex machining to ensure dynamic stability. Radial damping rings require full grooves for reliable axial location. These grooves have tight tolerances, requiring difficult and expensive machining, and can be difficult to clean, leading to contamination and premature bearing failure.
Secondly, good vibration isolation of a turbomolecular pump requires the mounting arrangement for the lower rolling bearing 10 to have a low radial stiffness. The overall radial stiffness of the mounting arrangement described above is the sum of the stiffness of the radial damping ring 24, and a further stiffness relating to shear deformation of the axial damping ring 28. However, the minimum hardness of elastomeric material restricts the minimum radial stiffness attainable using radial and axial elastomeric damping rings. Furthermore, over a period of time elastomeric material suffers from creep and stress relaxation under load. Elastomeric material also softens due to interaction with any lubricant discharged from the bearing. Softening of the axial damping ring increases the shear modulus of that damping ring and thus the effective radial stiffness of the mounting arrangement for the lower bearing, thereby undesirably increasing the vibration transmission to the pump housing with time.
The stiffness characteristics of elastomeric material are highly non-linear, and change markedly with time, temperature and interaction with lubricant, and so there is currently no accurate analytical model for predicting the stiffness and damping characteristics of the damping rings during use.
In addition, it is normal practice to carry out low-speed coarse balancing of the pump using a “slave” rolling bearing fitted to the rotor, so that the “final” rolling bearing that will be used during normal use of the pump is not subjected to excessive loads during balancing. When the final bearing is subsequently fitted to the rotor, there is however a degree of imbalance due to the differences in fit and raceway run out between the slave and final bearings. Consequently, high speed balancing must then be performed when the final bearing is fitted to the rotor. Since the stiffness characteristics of elastomeric material change with time, temperature and interaction with the bearing lubricant, the dynamic properties of the pump change during balancing, which leads to an increase in the duration of the balancing process to accommodate the variation with time of the mechanical properties of the mounting arrangement.
It is an aim of at least the preferred embodiments of the present invention to seek to solve these and other problems.